How do depth electrodes tell which hippocampus starts the seizures? · Grey Matter

Scalp EEG sees a temporal lobe seizure only once it has spread toward the surface, and by then both sides may be involved, so it can point to the wrong hemisphere or to both.

How do depth electrodes tell which hippocampus starts the seizures?

Scalp EEG sees a temporal lobe seizure only once it has spread toward the surface, and by then both sides may be involved, so it can point to the wrong hemisphere or to both. With an electrode in each hippocampus and others in the neighbouring cortex, the recording shows where the first change appears, often a burst of fast low-voltage activity at one contact, and how many seconds later it reaches the other side. If every recorded seizure starts in the same hippocampus, removing or ablating it has a good chance of stopping them; if they start on both sides, surgery on one side is unlikely to help and a stimulator is considered instead.